In my prior U.S. Pat. No. 3,748,916, granted July 31, 1973, there is disclosed a derailleur mechanism in which shifting from one sprocket to another having a numerically higher ratio to a numerically lower ratio is accomplished almost instantaneously by pinching a segment of the sprocket chain against the adjacent larger sprocket. This pinching action eliminates the need to move the derailleur past the larger sprocket and then return to the desired position. It now becomes practical to use a positively positioned derailleur mechanism as hereinafter described.
In the above mentioned patent, the actual means for laterally shifting the pivotal block is conventional, that is to say the pivot block is resiliently pulled against the action of a spring to one end of its potential path of travel by a single cable running to a control means. Conversely, the control means is released to allow the spring to pull the pivot block to the other extreme of its potential arc. This type of actuation was necessary in previous derailleur mechanisms because it was necessary to overshoot the larger sprockets in order to cause the chain to shift from smaller to incrementally larger sprockets. Upon completion of my work with the "pinch effect," I found that I no longer had to overshoot when shifting the chain from the smaller to the larger sprockets. According, I was able to concentrate upon an incrementally detented chain-shifting derailleur.